The present invention is directed, in general, to power conversion and, more specifically, to a controller for a power factor corrector and a method of operating the controller.
Power factor correction is a common requirement of many electronic systems today. To achieve a high power factor, switched-mode converters, such as boost converters, are often employed. Boost converters may be used to elevate an output voltage to a desired value greater than an input voltage. Boost converters can be operated in several different modes. In low to medium power applications, a discontinuous conduction mode (DCM) of operation is attractive because DCM operation uses a simple control circuit and does not need a complex snubber circuit. For higher power, power factor correction applications, a continuous conduction mode (CCM) of operation for the boost converter is normally used. However, CCM operation typically requires a more complex control circuit and a complex snubber circuit to operate satisfactorily.
Control of a conventional DCM boost converter employs essentially a constant duty cycle for the power switch of the boost converter. That is, the duty cycle of the power switch does not change significantly over a cycle of the AC input power. At xe2x80x9chigh linexe2x80x9d where the input voltage is at its highest RMS value, the slow discharge of the current through the boost inductor around the peak of the AC input voltage produces an excessive peak amount of input current. This excessive peak amount of input current can generate a high amount of total harmonic distortion (THD) for the input current.
For example, a boost converter operating in DCM with an input voltage of 265 volts and an output voltage of 400 volts may have an input current THD of around 50 percent. This amount of THD is generally not acceptable. To reduce the THD of the input current, some boost converters are operated in a critical conduction mode. That is, the power switch of the boost converter is closed a fixed time when the boost inductor current reaches zero each control period. However, such operation will cause the power switch to experience an unusually wide range of switching frequencies, requiring a high level of input filtering to meet electromagnetic interference (EMI) standards. A boost converter operating in DCM may also move into CCM operation under certain situations. In CCM operation, special care should be taken to account for excessive currents that may damage components of the converter that were designed to operate primarily in DCM.
Accordingly, what is needed in the art is an improved way to control PFC converters operable primarily in DCM that enhances the performance thereof without the complexity and cost penalties associated with conventional CCM converters.
To address the above-discussed deficiencies of the prior art, the present invention provides a controller for a power factor corrector (PFC), operable in a discontinuous conduction mode (DCM) and having a controllable switch, and a method of controlling the controllable switch. In one embodiment, the controller includes: (1) a drive circuit adapted to drive the controllable switch and (2) a current mode control circuit adapted to sense a current of the PFC and develop a current control signal based thereon. The drive circuit reduces a duty cycle of the controllable switch based on the current control signal to reduce input current distortion in the PFC.
The present invention introduces, in one aspect, a controller that modifies a duty cycle of the controllable switch (power switch) of the PFC based on an input current thereof. By judiciously reducing the duty cycle of the controllable switch, preferably around a peak of the input voltage, the peak input current to the PFC can be reduced, thereby affording a significant reduction in the total harmonic distortion (THD) of the input current. This judicious limiting of the peak value of the input current also makes the PFC more robust if its operation should change from a discontinuous conduction mode (DCM) to a continuous conduction mode (CCM) wherein damage to components may be more probable.
In one embodiment of the present invention, the controller further includes a frequency modulation control circuit adapted to sense a rectified output voltage of the PFC and develop a voltage control signal based on the rectified output voltage. The drive circuit modifies a switching frequency of the controllable switch based on the voltage control signal. In a related embodiment, the frequency modulation circuit includes a voltage divider for sensing the rectified output voltage. Of course, other sensing schemes may be employed to sense the rectified output voltage. The principle of frequency modulation of the controllable switch is described in detail in U.S. patent application Ser. No. 09/324,074, entitled xe2x80x9cFrequency Modulation Controller For Single-switch, Polyphase, DCM Boost Converter and Method of Operation Thereofxe2x80x9d to Hengchun Mao, filed Jun. 1, 1999. The above-listed application is herein incorporated by reference as if reproduced herein in its entirety.
In one embodiment of the present invention, the current mode control circuit includes a current sensor adapted to sense the current of the PFC and a filter adapted to develop the current control signal based on the current of the PFC. The current sensor may be used to sense current from any pertinent point in the PFC. The filter then smooths the current control signal to an appropriate degree to obtain a high frequency averaged current signal commensurate with the requirements of the drive circuit.
In one embodiment of the present invention, the drive circuit includes: (1) a voltage compensation circuit adapted to receive a signal representing an output voltage of the PFC and develop an intermediate signal therefrom and (2) a modulator, coupled to the voltage compensation circuit, adapted to develop a drive signal for the controllable switch based on the intermediate signal. Of course, other ways of developing the drive signal are well within the broad scope of the present invention.
In a related embodiment of the present invention, the voltage compensation circuit includes an error amplifier adapted to compare the signal representing the output voltage to a reference signal and develop the intermediate signal therefrom. The error amplifier accurately determines any deviation between the output voltage and the reference voltage to provide an appropriate intermediate signal for output voltage correction purposes.
In another related embodiment, the modulator includes a duty cycle comparator adapted to receive a ramp signal and the intermediate signal and develop the drive signal therefrom. The comparator accurately compares the input values of the ramp signal and the intermediate signal to determine the duty cycle for the controllable switch.
In one embodiment of the present invention, the PFC employs a topology selected from the group consisting of a boost converter, a buck converter, a forward converter, a flyback converter, and a SEPIC converter. Of course, other topologies are well within the broad scope of the present invention.
In one embodiment of the present invention, the PFC is further operable in a continuous conduction mode (CCM). The PFC may enter CCM, for example, during a peak of the input line voltage.
In another embodiment of the present invention, the PFC includes an input EMI filter. Depending on the topology used, the input filter may include a filter inductor, a filter capacitor or both the filter inductor and the filter capacitor. Of course, the use of any input filter topology is well within the broad scope of the present invention.
The foregoing has outlined, rather broadly, preferred and alternative features of the present invention so that those skilled in the art may better understand the detailed description of the invention that follows. Additional features of the invention will be described hereinafter that form the subject of the claims of the invention. Those skilled in the art should appreciate that they can readily use the disclosed conception and specific embodiment as a basis for designing or modifying other structures for carrying out the same purposes of the present invention. Those skilled in the art should also realize that such equivalent constructions do not depart from the spirit and scope of the invention in its broadest form.